The present invention relates to an over or furnace for the partial heat treatment of tools that have a clamping portion and a working portion, whereby the oven comprises a heating chamber that receives charges of tools and is equipped with a door and internal heating elements that irradiate heat for hardening tools under vacuum, at least one evacuating device, a quenching installation, and a charging platform for receiving tools. The clamping portion of the tools that is not to be heated is arranged inside the charging platform and the working portion of the tools that is to be heated is arranged outside the charging platform.
Ovens for a partial heat treatment of tools, for example, for unfinished twist drills made from high speed steel, are known. It is required that the cutting portions of such twist drills are fully hardened while their shaft portion, i.e., their clamping portion, they should be soft. The transition zone between the two portions should be minimal. This is achieved by converting the material of the cutting portion into austenite at temperatures from 1140.degree. C. to 1300.degree. C., depending on the high speed steel used, and subsequent quenching. At the same time, the shaft portion may not be heated above 850.degree. C.
In an oven of the prior art for the partial heat treatment the hardening step is carried out under vacuum. The tools, for example, unfinished twist drills, are inserted into a solid charging platform, usually made from steel and having a high heat-retaining capacity, and are subsequently introduced into a vacuum oven. Then the oven containing the charge of tools is evacuated and heated. The portions of the tools that are extending past the charging platform, i.e., the cutting portion of the twist drill, are heated by the radiation of the heating elements to the temperature at which the conversion to austenite takes place, while the portions inside the charging platform, i.e., the shaft of the unfinished twist drill, is shielded from the radiation. The large mass of the charging platform prevents the shaft portion from being heated to temperatures above 850.degree. C.
These known ovens have the disadvantage, that the heating and cooling period are very time consuming and also result in expansive transition zones between the hardened and the soft portions. Also, all the oven parts in the heating chamber such as the heating elements and the heating connections, the insulation and the charging platform must be heated during the initial heating step and cooled in the subsequent quenching step. This results in a very time consuming process and in high energy losses. The "efficiency" of such an oven with respect to the ratio of the loaded tools to the oven size and mass is extremely low. Also, the productivity of such an oven is very low, i.e., the output per hour versus the operating and purchasing costs is low, resulting in high manufacturing costs per piece.
It is therefore an object of the present invention to provide an efficient and productive oven for the partial heat treatment of tools under vacuum, which especially minimizes the transition zone between the hardened and the soft portions of the tool.